I believe this reflects a limitation of your DICOM images, and not a problem with dcm2niix. For GE equipment, we rely on the tags RTIA Timer (0021,105E) or Trigger Time (DICOM 0018,1060). Be aware that not all GE sequences populate these tags. Therefore, I believe that your raw data does not encode slice timing. You can verify this by examining the DICOM header. I would recommend dcmdump or gdcmdump to look at your header. However, you can also have dcm2niix report the DICOM header by including the logorrheic verbosity "-v 2". Since this mode generates so much output, you may want to isolate a single image from the series for this command. I suspect if you do this, you will find that your files do not contain either of these two timing tags. Your GE research collaboration manager can help you select EPI sequences that generate these tags.